Medical devices may be coated so that the surfaces of such devices have desired properties or effects. For example, it may be useful to coat medical devices to provide for the localized delivery of therapeutic agents to target locations within the body, such as to treat localized disease (e.g., heart disease) or occluded body lumens. Localized drug delivery may avoid some of the problems of systemic drug administration, which may be accompanied by unwanted effects on parts of the body which are not to be treated. Additionally, treatment of the afflicted part of the body may require a high concentration of therapeutic agent that may not be achievable by systemic administration. Localized drug delivery may be achieved, for example, by coating balloon catheters, stents and the like with the therapeutic agent to be locally delivered. The coating on medical devices may provide for controlled release, which may include long-term or sustained release, of a bioactive material.
Aside from facilitating localized drug delivery, medical devices may be coated with materials to provide beneficial surface properties. For example, medical devices are often coated with radiopaque materials to allow, for fluoroscopic visualization while placed in the body. It is also useful to coat certain devices to achieve enhanced biocompatibility and to improve surface properties such as lubriciousness.
Coatings have been applied to medical devices by processes such as dipping, spraying, vapor deposition, plasma polymerization, spin-coating and electrodeposition.
The spray-coating method has been used because of its excellent features, e.g., good efficiency and control over the amount or thickness of coating. In the spin-dipping process, a medical device is coupled to a spinning device, and then, while rotating about a central axis, the medical device is dipped into a coating solution to achieve the desired coating.
In addition to the spray coating and spin-dipping methods, the electrostatic deposition method has been suggested for coating medical devices. For example, U.S. Pat. Nos. 5,824,049 and 6,096,070 to Ragheb et al. mention the use of electrostatic deposition to coat a medical device with a bioactive material. In the conventional electrodeposition or electrostatic spraying method, a surface of the medical device is electrically grounded and a gas may be used to atomize the coating solution into droplets. The droplets are then electrically charged using, for example, corona discharge, i.e., the atomized droplets are electrically charged by passing through a corona field. Since the droplets are charged, when they are applied to the surface of the medical device, they will be attracted to the surface since it is grounded.
Another method of coating a device can be achieved with electrohydrodynamic spraying. Using this method, a gas is not needed to disperse the coating. Electrohydrodynamic coating is accomplished by forcing a compatible solution through a nozzle assembly that has been electrically charged. The coating solution passes through the charged nozzle where it is electrically charged. As the solution exits the nozzle, the solution atomizes as the charged particles repel each other. This action forms the spray mist. The charged particles are attracted to the device to be coated since the device is connected to an electrical ground.
Devices may be coated by a gas assisted spraying process. A polymer/drug combination may be dissolved in a solvent mixture. The solution may be sprayed onto the devices and a polymer/drug film may be formed when the solvents evaporate. The ability to apply thin coatings on products may be limited by the capabilities of a gas assisted spraying process. The coating may flow on the medical device prior to drying, thereby creating an uneven concentration of bioactive agent on the surface of the device. A gas assisted spraying process may have a high variability for thin coatings.
Conventional methods of coating stents or devices with a drug-polymer layer, such as spraying or dipping, may require a solution of the drug-polymer to physically wet the surface of the stent. Spraying or dipping may cause uneven and unpredictable wetting, and distribution and evaporation of the solvent molecules may result in a non-uniform coating. The drying of the coating may lead to cracking and/or points of stress in the coating. A non-uniform coating may lead to the unit failing agent release requirements, drug uniformity and coating thickness specifications.
Conventional drug-eluting stents may include paclitaxel particles incorporated into a polymer matrix. The polymer may be hydrophobic and may entrap a large amount of drug (for instance, possibly over 95% in the 8.8% formulation of Boston Scientific's TAXUS SR drug eluting stent) that may not be subsequently delivered when implanted in a lumen of a human body. There may be government regulatory issues regarding sequestered drug that may remain entrapped indefinitely, as well as safety concerns.
There is, therefore, a need for a cost-effective method of coating devices that results in uniform, defect-free coatings and uniform drug doses per unit device. The method would provide better control of the agent release profile of the device, including releasing a higher percentage of the bioactive agent. The method would also improve the quality of the coating of the device by removing defects, cracks and stress points in the coating. Each of the references cited herein is incorporated by reference herein for background information.